Theory of Plate Tectonics: The Earth’s crust is divided into
pieces and those pieces are in constant motion.

1. The
continents look like puzzle pieces that could be put together.

2. Identical fossils of extinct
herds and identical rock outcrops are found on both sides of the ocean.

3. Earthquakes and volcanic
activity almost all seem to happen along specific lines in the earths crust.

4. Magnetic Reversals in the Earths
Crust: Mafic igneous rock as it cools has a high iron content and the iron
atoms will align themselves with the earth’s magnetic field as it cools. Spreading
out from the ridges of the ocean are mirrored patterns of the earth’s magnetic
field reversals.

Earth’s Crust:

Lithosphere: Crust plus the top
cooler part of the upper mantle. The top part of the lithosphere is brittle the
bottom part is fairly rigid but bends and flexes under immense force.

Asthenosphere: Between the
lithosphere and the mantle the asthenosphere is a thin liquid layer that allows
plates to flow on it.

Oceanic Crust: underneath the sea
floor, is thin, and made of high density rocks (basalt and gabbro)

Continental Crust: under the
continents, is thicker and less dense allowing it to float higher on the
mantle, and made of low density rocks like granite.

Theories for why Plates Move:

Convection: The idea that head
rises and cool sinks. So the deeper rocks in the mantle are rising cooling and
then falling. And while they do this they create a force that pulls the plates
around.

Doesn’t work because of all the
random ways the continents seem to be moving.

Gravitational Sliding: (ridge-push
force) The mid oceanic ridges are raised portions of the crust. As new crust
forms at the top they begin to slide downward and push the plate in front of
them.

If this theory is correct it would
mean that sea floor spreading causes plate tectonics.

Slab-Pull force: A denser plate
will fall under (subduct) a less dense plate. A sinking piece of oceanic crust
will work like an anchor and pull the rest of the plate along with it.

Speed of a Tectonic Plate:

1-15 cm per year

global
positioning system (GPS) is now used to track the motion of the plates.

Plate Boundaries:

Earthquake Belts: Plate boundaries are
located by the mapping of earthquake foci (the spot where the earthquake
originates.

mid-oceanic ridge: where the
divergent boundary has risen about 2 km above the ocean floor in long straight
lines.

new ocean floor is created right at the boundary between the two
separating plates.

As the plates spread apart a gap
does not form, instead hot asthenosphere rises (magma) to fill the gap. Cools
and forms new oceanic crust.

As the new crust moves away it
will cool more and become more dense and sink lower. You can actually tell the
age of the ocean floor by how deep it is, the older it is the denser it is the
deeper it sinks.

symmetrical: the eastern side of a
ridge will look just like the western side since they formed at the same
time.

fracture zones: often a ridge will
slide perpendicular to it direction (jog left or job right). This is where
earthquakes have occurred along the ridge.